Energy consumption in wastewater treatment plants (WWTPs) largely depends on the treatment processes. Traditional technologies, for example, their energy consumption in domestic wastewater treatment (DWW) could be sorted: constructed wetlands (0.253 kWh/m³) < anaerobic/anoxic/oxic process (0.267 kWh/m³) < conventional activated sludge process (CAS, 0.269 kWh/m³) < anoxic/oxic process (0.283 kWh/m³), oxidation ditch (0.302 kWh/m³) < sequencing batch reactor (0.336 kWh/m³) < membrane bioreactor (MBR, over 0.370 kWh/m³) (Yang et al., 2010; Krzeminski et al., 2012). Additionally, shortcut nitrification-denitrification could theoretically reduce 25% oxygen (O₂) consumption and 40% organic carbon (OC) consumption, and anammox could theoretically respectively decrease 62.5% O₂ consumption and 100% OC consumption (Adams et al., 2022), possessing a high potential of energy minimization and GHG emission reduction. Autotrophic denitrification (ADN) utilizes inorganic reductants, e.g., hydrogen (H₂) (Deng et al., 2020b), iron (Fe⁰/Fe²⁺) (Peng et al., 2019) and sulfur compounds (S⁰, S^2- and S₂O₃ ^2-) (Di Capua et al., 2019), as electron donors for denitrification, reducing 100% of OC consumption and corresponding CO₂ emission. Moreover, ADN utilizes CO₂ as a carbon source (Di Capua et al., 2019), obtaining the advantage of CO₂ caption in denitrification.

Process optimization could further minimize energy consumption and GHG emission. In traditional WWTPs, aeration consumes almost 60% of the total energy (Lozano Avilés et al., 2019). Thus, many studies have focused on reducing energy consumption in aeration through aeration optimization, e.g., the propose of ammonia-N-based feedback aeration control strategy (Sun et al., 2016) and the air supply process optimization through modeling (Jiang et al., 2023). For instance, Jiang et al. (2023) applied a comprehensive model composed of a mass flow analysis module, an oxygen transfer rate module and an aeration optimization module to develop an aeration optimization strategy to reduce the energy consumption of a full-scale anaerobic/anoxic/oxic system. It reduced the energy-saving potential of blowers by 14.1% with an improved air supply prediction accuracy (Jiang et al., 2023). In terms of GHG emission, proper optimization of denitrification processes, e.g., the Fe⁰-/H₂-supported ADN, could also reduce over 90% of the N₂O emission (Li et al., 2017; Deng et al., 2020a). In addition to N₂O, process optimization could also reduce CH₄ production during H₂ generation from wastewater, e.g., that in microbial electrolysis cells (MECs) (He et al., 2022).

Technology advancement mainly involves the innovation, strengthening and combination of the existing technologies, which also showed potential for reducing energy consumption and GHG emission in wastewater treatment.

In carbon (i.e., COD, mg/L) to nitrogen (i.e., total nitrogen, mg/L) (C/N) ratio wastewater treatment, traditional biological treatment processes could be innovated by coupling ADN, reducing CO₂ emission compared to dosing external OC (e.g., CH₃OH and CH₃COOH) (Deng et al., 2019). For instance, Fe⁰-ADN has been successfully introduced into the anaerobic/anoxic/oxic process (Peng et al., 2020) and constructed wetland (Deng et al., 2020b), and achieved efficient N removal. In ADN, OC addition could be an innovative strategy for GHG emission reduction. Deng et al. (2022) reported that OC dose at 0.25 mg-C/mg-N in Fe⁰-ADN could reduce N₂O emission by over 80%.

In AOPs, innovations could be focused on developing and applying catalysts to reduce chemical and energy consumption (Wu et al., 2021). Ozonation as an example, Lakshmi et al. (2018) and Deng et al. (2021) respectively developed FeO_x-doped granular activated carbon (Deng et al., 2021a) and TiO₂/Fe₃O₄-composited graphene oxide (Jothinathan et al., 2021) to build catalytic ozonation processes, and both obtained enhanced performance and reduced ozone (O₃) consumption, reducing the energy consumption.

In membrane processes, innovations should be focused on membrane fouling mitigation, e.g., membrane material and operational mode development, to improve energy efficiency (Deng et al., 2022). Deng et al. (2022) introduced iron-carbon galvanic cells (Fe⁰||C) into MBR, confirming a 22% improvement in phenolic-compounds removal and a 71% membrane fouling extension. Wang et al. (2021) developed a novel vibrating MBR for DWW treatment. It showed superior energy efficiency for fouling control and saved 51.7%–78.5% energy compared to the conventional air-sparging MBR (Wang et al., 2021).

Beyond innovation and strengthening, process combination is also an alternative. Deng et al. (2021b) combined ozonation with MBR for phenolic wastewater treatment. The pretreatment by ozonation reduced the acute biotoxicity and increased the 5-day biochemical oxygen demand (BOD₅) to chemical oxygen demand (COD) ratio (BOD₅/COD), contributing to performance enhancement and membrane fouling mitigation. This combination decreased the membrane fouling rate by over 88% and reduced the O₃ consumption of ozonation by over 50% (Deng et al., 2021b).

Various technologies have been devised for nutrient recovery from wastewater (Wu and Vaneeckhaute, 2021), including physicochemical technologies (e.g., crystallization and stripping), anaerobic technologies, photo-based biotechnologies (PBTs, e.g., microalgae-based technologies and photobacteria-based technologies) and METs.

Anaerobic technologies, e.g., AnMBR, also enables nutrients together with energy generation (Wang et al., 2022). However, coupled techniques, e.g., MgNH₄PO₄•6H₂O (MAP) formation, NH₃ stripping-absorption, membrane distillation or PBTs, are required for nutrient recovery from AnMBR-treated effluent. PBTs are promising alternatives for nutrient recovery, capturing CO₂ from the air and producing a small carbon footprint (You et al., 2023). The combined AnMBR-PBTs process enabled the efficient simultaneous recovery of energy and nutrients and addressed the problem of microalgae and photobacteria harvest in traditional PBTs (Li et al., 2022). However, PBTs are sensitive to toxicity from heavy metals or organics (Tiang et al., 2020). Hence, they are mainly considered for DWW treatment, whereas the development of toxicity resistance photobacteria and corresponding treatment systems are in need to widen the application of PBTs (Li et al., 2022; You et al., 2023).

METs, especially MRCs, also could recover nutrients during energy generation and exhibited a much higher toxicity tolerance than PBTs. Huggins et al. (2016) developed an MRC based on a tubular overflow style MFC for resource recovery from actual industrial wastewater. The system achieved a COD removal rate of 0.55 kg/(m³·d) and a maximum power density of 6 W/m³, and recovered 2 g P/kg and cathode 1 g K/kg on the cathode in 20 days’ continuous operation (Huggins et al., 2016). METs are constructed from low-cost, biogenic and biocompatible materials that recover organic carbon and nutrients (Goglio et al., 2019b). With the development of MRCs, the electrodes saturated by micro- and macro-nutrients could be reused as organic-mineral fertilizers (Goglio et al., 2019a). The concept of MRCs also greatly increased the possibility of scaling up of METs (Deng et al., 2023). Thus, MRCs are potential biotechnology for energy and nutrient recovery from wastewater, and efficiency improvement and up-scaling are the future perspectives.

MRCs also enable the recovery of minerals, especially those that can be utilized as nutrients (e.g., K and Mg), and the products could be applied as organic-mineral conditioners or fertilizers for agricultural soil improvement (Goglio et al., 2019a). Nevertheless, many of the other minerals, e.g., the toxic heavy metals, are not suitable to be recovered by MRCs due to their side effects in agricultural application (Goglio et al., 2019b). Otherwise, they can be effectively recovered by traditional adsorption, sedimentation, electro-deposition and electrodialysis processes (Taghvaie Nakhjiri et al., 2022). As MRCs have the risk enriching toxic minerals, the risk of the products to agricultural soil should be evaluated prior to application.

Several technologies have been developed and applied for the valuable salts recovery from industrial and domestic wastewater, such as the thermal process (e.g., multi-effect distillation and multi-stage flash distillation), electrochemical process, and membrane process (e.g., high-pressure reverse osmosis and forward osmosis) (Panagopoulos and Giannika, 2022b). Panagopoulos et al., 2022b treated brine with high-pressure reverse osmosis (HPRO) and obtained a salt recovery of up to 0.435 under the optimum conditions. In addition, the use of energy recovery systems has reduced energy consumption by about 26% (Panagopoulos, 2022). Although HPRO is a promising technology for saline wastewater and salt recovery, its performance is also limited by problems such as scaling, membrane fouling and membrane compaction (Panagopoulos, 2022). Future research should focus on the development of new membrane materials and the stability of the membrane treatment process.